Hydrofluoroalkanes can be prepared by reaction of an appropriate chlorinated precursor with hydrogen fluoride, as disclosed, for example, in Patent Applications EP-A1-0,699,649 and WO-A1-97/15540 (on behalf of Solvay) and in Patent Application WO-A1-97/05089. In such a process, at the outlet of the reactor, the mixture of reaction products comprises, in addition to the desired hydrofluoroalkane, hydrogen chloride originating from the removal of the chlorine atom or atoms from the starting chlorinated precursor, hydrogen fluoride, chlorofluorinated intermediates, generally unconverted chlorinated precursor, possibly inert diluents, and various byproducts in small amounts. Given that the operation is usually carried out with an excess of hydrogen fluoride with respect to the chlorinated precursor, unconverted hydrogen fluoride is generally present in the mixture of reaction products. Whereas the majority of the constituents of the mixture of reaction products can be easily and completely separated by distillation, complete separation between the hydrofluoroalkane and the hydrogen fluoride is generally very difficult to achieve by distillation, this being because these compounds often form azeotropic mixtures.
Patent Application WO-A1-97/05089 discloses a process for the purification of hydro(chloro)fluoroalkanes (in particular 1,1,1,3,3-pentafluoropropane or HFC-245fa) from azeotropic mixtures with hydrogen fluoride by an azeotropic distillation technique comprising two successive stages of distillation at different temperatures and at different pressures.
However, this azeotropic distillation technique exhibits the disadvantages of requiring a large difference in temperature or in pressure between the two columns, so as to have available a sufficient separation potential (difference in composition between the low pressure/temperature azeotrope and the high pressure/temperature azeotrope), and of producing a high recycling flow rate between the two columns.
Patent Application WO-A1-97/13719 discloses a process for the separation and recovery of hydrogen fluoride from its (azeotropic) mixtures with, inter alia, hydrofluoroalkanes comprising from 1 to 6 carbon atoms (in particular HFC-245fa). The mixture is brought into contact with an alkali metal fluoride (in particular, potassium fluoride or caesium fluoride) solution and the organic phase is separated from the phase comprising the hydrogen fluoride and the alkali metal fluoride.
Through this known process, contamination of the organic phase by the potassium fluoride or caesium fluoride and the risk of decomposition of the hydrofluoroalkanes which this contamination might bring about may be feared. Moreover, these alkali metal fluorides, and more particularly caesium fluoride, are very expensive.